In recent years, demand for use of communication systems that use high frequency waves is increasing. The high frequency communication systems utilizes a millimetric-wave, and the high frequency communication systems cover frequency band of a broad range, thereby being designated as an Ultra Wide-Band system or the like. In addition, demand for passive millimetric-wave imaging systems is also increasing in an area of sensing system.
In the course of developing the millimetric-wave sensing systems, an antenna for covering the broad band and electric circuits for signal processing are required. The antenna for the broad band is, for example, a waveguide type antenna (a horn antenna). The electric circuit for the signal processing is, for example, a microstrip type circuit. The millimetric-wave captured by the horn antenna is sent to a waveguide-microstrip conversion system before being supplied to the microstrip type circuit.
Conventional millimetric-wave antennas and related circuits for the broad band are disclosed in Japanese Patent Documents JP-A-H11-163636 and JP-A-H11-330846. The antenna disclosed in these documents are planar antennas, and the planar antennas can be formed on the same substrate as the circuit for millimetric-wave detection. Therefore, there is no need for the planar antenna to have the waveguide-microstrip conversion system that is conventionally required for waveguide antennas.
In addition, it is disclosed in IEEE Transactions on Antennas and Propagation Vol. 38, No. 9, September 1990, pp 1473-1482, as another technology of combination of the waveguide horn and the planar antenna. In this structure, the planar antenna has the shape of a horn antenna and a membrane (a film) is established in the horn antenna perpendicularly in a propagation direction of a millimeter wave. In this manner, compactness of a depth direction of the structure is substantiated.
However, the technology disclosed in Japanese Patent Documents JP-A-H11-163636 and JP-A-H11-330846 uses the antenna of an end-fire type that has a great dimension in a direction of depth of the waveguide. Therefore, the high frequency module with the end-fire type antenna has to be housed in a long package of a receiver unit, thereby preventing downsizing of the package.